1. Field of the Invention
The present invention relates to devices for detecting the presence of an object, and more particularly to normalizing performance characteristics of such sensors to facilitate replacement of one sensor with another sensor.
2. Description of the Related Art
Proximity sensors are commonly used along assembly lines to detect the presence of a workpiece passing nearby. The presence of a workpiece activates equipment that perform manufacturing operations.
U.S. Pat. No. 6,650,111 discloses an inductive proximity sensor for detecting presence of a metal object. The proximity sensor includes a drive circuit connected to a transducer coil to generate an oscillating signal. That signal changes when a metal object is adjacent to the transducer coil and a characteristic of the oscillations changes in response to the distance between the metal object and the transducer coil. A processing circuit detects that characteristic of the oscillations to produce an indication of the presence of an object.
In this type of proximity sensor, a resonant circuit is excited by a short duration voltage pulse, after which the resonant circuit is allowed to ring down whereby the oscillations decay exponentially. The rate of the exponential signal decay is proportional to the quality factor Q of the resonant circuit, which varies in relation to the distance to a metal object. Thus the number of peaks of the decaying signal that exceed a predefined level can be used to indicate the presence of a metal object within a given distance from the sensor. A particular model of proximity sensor is designed to emit an active output signal, indicating the object's presence, when the pulse count is within a predefined range of values.
The performance characteristics of the resonant circuit and the proximity sensor as a whole vary from device to device of the same model. As a consequence, each proximity sensor had to be adjusted during installation to respond to objects within the proper range of distances for that application. With some types of sensors, the installation setup involves physical adjustment of the sensor mounting to the piece of equipment on which it is being used. For other sensors, the setup is accomplished by electrically adjusting the signal processing circuit. In either case, some amount of manual effort by a technician is required to configure the sensor's operation. Even greater setup effort often is required when a sensor replaced one of a different type having performance characteristics which are more dissimilar.
Therefore, it is desirable to provide a mechanism that facilitates replacement of sensors.